1. Field of the Invention
The invention concerns steels with an ultra-high carbon content, or Ultra High Carbon (UHC) steels, of reduced density and high sealing resistance, and the production of components by semi hot-forming.
2. Description of the Related Art
UHC steels have already been known for a long time. They were developed particularly having regard to their superplastic properties. Superplastic forming takes place within a narrow process window of temperature and deformation rate (elongation rate (ε′)). During superplastic deformation elongations of a few hundred up to 1000% can be reached. Typical for this are a deformation temperature above around 50% of the melting temperature (ideally in the area of the α→γ transformation) and a very low deformation rate of about 10−2 to 10−5 s−1. If the respective optimum temperature and/or deformation rate are exceeded, the structure required for the good mechanical properties is destroyed. The ideal speeds for superplastic deformation is therefore substantially below the limit of industrial acceptability for mass-produced products, which is approximately 0.1/s.
Unalloyed UHC steels, as known for example from U.S. Pat. No. 3,951,697, show only a slight superplastic effect, since the structure is unstable against grain growth. U.S. Pat. No. 4,448,613 describes methods for producing the superplastic structure in UHC steels. The control of the superplastic structure in UHC steels with small amounts of Cr, Mn and Si alloying additions is also described.
In U.S. Pat. No. 4,533,390 it is proposed, in a UHC steel with very high Si content (3 to 7%), to increase the A1 temperature, stabilize the structure against grain growth and improve the superplastic properties by means of alloying additions of Cr, Mo, W, Ti and their combinations. The high Si contents make these steels very brittle under ordinary service conditions.
U.S. Pat. No. 769,214 describes UHC steels with a high fraction of Al (preferably 0.5 to 6.4%). The aim is to produce good superplastic properties, in particular good deformability under superplastic conditions, and resistance to oxidation. To stabilize the structure alloying additions of Cr and/or Mo are made. With an Al fraction higher than 6.4% the hot and cold deformability were found to decrease markedly. The preferred UHC steels have Al contents lower than 6.4%.
The deformability of the material is particularly important for the economy of the forming process. Good deformability means being able to reach a high degree of deformation without damage to the component, low yield stress during deformation, and the lowest possible deformation temperature. Only then can components even of complex shape be produced in a few, inexpensive deformation steps.
Although cold forging (cold extrusion molding) can achieve great dimensional accuracy, good surface quality and high component strength (due to work hardening), it suffers from the disadvantages associated with sometimes much higher necessary deformation forces.
During hot forging (at around 1100° C. to 1250° C.) the materials show high deformation capacity (suitable for components of complex shape), but the dimensional accuracy is poor and the surface quality not as good. A particular disadvantage is the high thermo-mechanical loading of the die and the correspondingly severe die wear. Forming pressures in the high-temperature range, for example at forging temperature, result in high tooling costs since either severe wear takes place or expensive high-temperature dies have to be used. Moreover, for reasons of cost the blanks being deformed are processed in air and thereby damaged by oxidation. In steels this leads for example to scaling. Before the further processing of the components so produced, finish machining has to be carried out at least at the surface. Consequently, near-net-shape production of components is possible only to a very limited extent at such temperatures.
Also very important in the context of mass production, particularly in the automotive industry, is high processing speed in the deformation process. Thus, the very low deformation rates in superplastic forming are not acceptable for the mass production of components.
With the known UHC steels having a low Al content, at the usual deformation temperatures of around 750° to 950° C. considerable scaling is to be feared, which can lead to additional machining costs. These steels are not suitable for lightweight construction.